What the Earth is Made Of

The earth condensed from a dust cloud some five thousand million
years ago. The dust contained a wide range of chemical elements,
being especially rich in iron, nickel, silicon, magnesium and
oxygen. As a consequence, the earth consists predominantly of
silicate systems with a metallic core. (The gravitational forces
acting on the heavier metallic elements Fe and Ni lead to their
concentration in the centre of the earth.) A schematic illustration
of the structure of the planet is shown on the left. It comprises
three main components: the core, the mantle and the crust. The
former is, as noted, metallic, consisting of an alloy (a
crystalline substance containing two types of metal, in this case
Ni and Fe). The mantle can be subdivided into upper and lower
sections. The lower region consists mainly of the mineral magnesium
silicate(MgSiO3), but at the immense pressures present
at this depth an unusual structure is adopted, in which silicon is
six-fold coordinated and the material adopts a structure similar to
that of perovskite as shown. This high density structure under
normal pressures is unstable and will transform to a structure in
which the silicon is tetrahedrally coordinated. At the lower (but
still very large) pressures in the upper mantle, tetrahedrally
coordinated silicate structures of composition
Mg2SiO4 dominate. This interesting compound
shows different structures at different pressures, with the spinel
structure (in which Si2O7 units are present)
adopted at high pressure (greater depths) and the olivine structure
at the lower pressures in the outer portion of the mantle. All
these mantle minerals contain high concentrations of iron (the
ratio of magnesium to iron is typically 10 to 1); the iron is
easily accommodated in the structure simply by replacing magnesium
atoms.

MgSiO3 Perovskite.

The thin crust of the earth on which we live is a rich and
complex assembly of minerals, again dominated by silicate
chemistry. The range of geological processes from vulcanism to
erosion have provided a vast and continuing experiment in solid
state chemistry that scientists are only beginning to understand.
Similarly, understanding of the physical properties of the mantle
minerals is yielding an understanding of how the planet works as a
whole.

MgSiO3 Ilmenite.

So inorganic chemistry and crystallography have given us a
detailed understanding, again of the arrangement of atoms, in the
minerals from which our planet is made; in just the same way,
organic and bio-chemistry have allowed us to understand at the
atomic level the materials of which our bodies and all living
things are constructed.

DEFECTS

The perfect crystalline solid represents an extreme of complete
order. Such a state is unattainable except in the hypothetical
absolute zero. The drive in nature towards disorder - towards
higher entropy - means that even if the energy cost is high,
elements of disorder must always be present in ordered crystals. Of
course at sufficiently high temperatures, the order is lost
entirely and the crystal melts. But crystals below their melting
point always contain some disordered structures known as defects.
And these species are not a mere curiosity. They can exert a
controlling influence on many of the most important properties of
the crystal, for example its mechanical strength and the rates at
which atoms can diffuse through the crystal.

The simplest type of defects are those in which atoms are simply
missing from normal sites or additional ones inserted. Both provide
effective ways of promoting atomic migration properties. The empty
sites (vacancies) allow neighbouring atoms to jump into them, while
the extra 'interstitial' atoms can therefore move through the
crystal.

A major source of defects in almost all crystals are impurities.
Even with the most rigorously careful methods for preparing pure
crystals, impurities are inevitably present. They commonly occupy
the regular sites of the crystal, and they can have dramatic and
profound effects on the properties of the crystal. The classic
example is the semiconductor material silicon: when small
concentrations of phosphorus impurities are introduced, the
phosphorous atoms replace the silicon and form four covalent bonds
to neighbouring silicon atoms. But phosphorus has one more electron
compared with silicon and the extra electron is easily removed
(ionised) from the phosphorus; it may migrate through the crystal
and will therefore enhance the electrical conductivity. Silicon is
a semiconductor and when phosphorus impurities are included (or
doped) into the material, it becomes an 'n-type' semiconductor ('n'
because the excess negatively charged electrons carry the current
within the material). Alternatively we can dope the silicon with an
element like boron which has one less electron. Again the boron
substitutes for the silicon but to establish its covalent bonding
pattern with the surrounding atoms, it needs to grab one electron.
So it pulls one out of the surrounding crystal leaving behind a
deficiency of electron. This 'missing electron' really behaves like
a quasi-particle. It too can move through the crystal (it is
really, of course, other electrons that are moving, but it works to
think in terms of the 'hole' moving) and behaves as though it had a
positive charge (a missing negative particle is a positive
species). Once again the electrical conductivity of the
semiconductor is enhanced but the material is now a 'p-type'
semiconductor because of the excess of positively charged
holes.

We can therefore 'tune' the electrical properties of silicon by
adding these very low levels of impurities (typically less than one
in a million silicon atoms will be replaced by the impurity). And
fascinating phenomena follow when 'p' and 'n' materials are put
together. thus a p/n junction has 'rectifying' action. It only
allows electrical current to flow in one direction - a vital
component in electrical circuitry. Indeed, the discovery of the
rectifying action of p/n junctions by Bardeen and Shockley heralded
the modern electronic age. The computing technology which has so
transformed our lives in recent years, the computers that were used
to generate the images here, all rely on the possibility of
controlling the electrical properties of crystalline silicon by low
levels of impurities.

Mg2SiO4.

More complex but equally important types of disorder are present
in 'line' defects known as 'dislocations', which involve a
localised fault in the mode of packing of the crystal, for example
the incomplete insertion of an extra layer of atoms. These species,
which may be created when the crystal grows and which are
introduced by heat or mechanical damage, drastically influence the
mechanical behaviour of the material; they allow the material to
flow and distort and that presence in high concentrations can lead
to the failure of the material.

The science of defects in solids has progressed enormously over
the last forty years. Indeed, the whole field of order and disorder
in solids has now reached a sophisticated level of understanding.
Interestingly, some solids tolerate very high levels of disorder
while remaining crystalline, while for others only low
concentrations of defects are present even at the melting point.
But once again, we can understand these contrasts in terms of the
balance between the energy required to create the disorder and the
entropy that is gained on its creation.